i6o LIFI-: : OUTLINKS OF GENERAL BIOLOGY 



droplet of phosphate of ammonia, a remarkable change may be 

 watched under the microscope, passing from one coloured cell to 

 another down the stalk ("cytoplasmic aggregation"). 



The tentacles are more or less indifferent to falling drops of rain; 

 but, as Darwin showed, they are extraordinarily sensitive to solid 

 particles and to certain chemical stimuli. A morsel of human hair, 

 weighing only :>.\^n of a grain, and this largely supported by the 

 viscid secretion, suffices to induce movement. This seems almost 

 hjTX'rsensitive; l)ut it is doubtless important that the plant should 

 react to the light touches of minute midges and the like. Darwin's 

 e.xiHTiments in connection with chemical stimulus are very interest- 

 ing. He treated sixty-one leaves of Drosera with droplets of non- 

 nitrogenous solutions, such as sugar and olive oil, and in not a 

 single case was a tentacle inflected. But when sixty-four other 

 leaves were tried with various nitrogenous fluids, such as milk, 

 albumen, mucus, beef-extract, sixty-three showed marked inflection 

 of the tentacles. And when he took twenty-three of the leaves that 

 had served for the first experiment, and treated them with nitro- 

 genous particles or fluids, the tentacles of all save a few were dis- 

 tinctly inflected. Thus a slight trace of nitrogenous substance on 

 the surface of the insects that light on the leaves will serve to 

 induce a movement of the tentacles. Darwin showed that there was 

 ]x*culiar sensitiveness to salts of ammonia, esjx'cially phosphate, 

 nitrate, and carbonate, even in infinitesimal quantities. Thus the 

 immersion of a leaf in a solution of phosphate of ammonia, so 

 weak that each gland could only absorb j 0000000 of a grain, was 

 sufficient to produce complete inflection of the tentacles. 



In some cases the tentacles respond to the contact of inorganic 

 particles, which is obviously unprofitable; but it is interesting to 

 notice that the reaction does not follow unless the contact is repeated 

 twice or thrice, which is what occurs when an insect touches the leaf 

 with its chitin-covered limbs. 



Darwin noticed that the tentacles of the sundew moved inwards 

 when the temperature was suddenly increased; and this may 

 ptThaps make for efficiency on sunny days when insects are most 

 in evidence. This sensitiveness to warmth has been confirmed, with 

 th<> interesting detail that it is paralysed when the plant is supplied 

 with water rich in lime-salts, which are absent from the ordinary 

 moorland habitat. Neger calls attention to this as an instance of 

 the delicacy of the jihysiological poise in some organisms. The 

 nutritional change in this case is but a small one, but it brings about 

 a very striking alteration of habit. 



The leaves of the sundew .seem to have some attraction for insects, 

 but whether this is due to their colour, their glistening secretion, 

 or their odour, or to all three, remains uncertain. The exudation of 

 the viscid secretion may be slight at first, and involve only one or 



